wendelin: Adjust to recent ipython-notebook section renaming

Commit cee110b2 (IPython Notebook: Fixing coding crimes for section
names) changed IPython notebook section name to use '-' as word
delimiter but forgot to update users, and this way wendelin build
started to fail:

    INFO While:
    INFO   Installing.
    INFO   Getting section ipython_notebook.
    INFO Error: The referenced section, 'ipython_notebook', was not defined.

Fix it. (And I've made sure with whole-tree git grep that there is no
more ipython notebook users except wendelin in whole slapos.git so far)

/reported-by @Tyagov
/cc @tiwariayush
/reviewed-by TrustMe

software/wendelin/software.cfg

@@ -21,7 +21,7 @@ parts +=
   wendelin.core
   matplotlib
   fluentd
-  ipython_notebook
+  ipython-notebook
 
 [eggs]
 initialization =